This chapter covers the following topics that you need to master for the CCNP BCMSN exam:
■ Wireless LAN Basics—This section discusses wireless networks as they compare to wired Ethernet networks.
■ WLAN Building Blocks—This section covers wireless service sets in addition to wireless access points and their coverage areas.
■ An Introduction to Wireless LAN RF—
Wireless networks use radio frequency (RF) transmissions as their communications medium. This section covers basic RF theory and the terminology that is used to define and describe wireless operation. This section also discusses wireless LAN antennas and their applications.
■ WLAN Standards—This section describes the IEEE standards; specifically, the 802.11b, 802.11g, and 802.11a standards that are used in wireless LAN environments.
C H A P T E R 17
Wireless LAN Overview
Switched networks generally form the foundation of an enterprise network. Connectivity is offered from the core layer downward to reach end users located at the access layer.
Traditionally, these end users have used wires to connect to the access layer.
Wireless networks allow the access layer to be extended to end users without wires. By designing and placing wireless LAN devices across an entire area of the network, end users can even become mobile and move around without losing their network connections.
This chapter presents an overview of the technologies used in wireless LANs. By becoming familiar with some basic wireless theory, you will be able to understand, design, and use wireless LAN devices to expand your switched network to reach wireless users.
“Do I Know This Already?” Quiz
The purpose of the “Do I Know This Already?” quiz is to help you decide what parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.
The quiz, derived from the major sections in the “Foundation Topics” portion of the chapter, helps you determine how to spend your limited study time.
Table 17-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?”
quiz questions that correspond to those topics.
Table 17-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
Foundation Topics Section
Questions Covered in
This Section Score
Wireless LAN Basics 1–2
WLAN Building Blocks 3–4
Wireless LAN RF 5–9
WLAN Standards 10–12
Total Score
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1. Which one of the following standard sets is used in wireless LANs?
a. IEEE 802.1
b. IEEE 802.3
c. IEEE 802.5
d. IEEE 802.11
2. Which one of the following methods is used to minimize collisions in a wireless LAN?
a. CSMA/CD
b. CSMA/CA
c. LWAPP
d. LACP
3. A wireless scenario is made up of five wireless clients and two APs connected by a switch.
Which one of the following correctly describes the wireless network?
a. BSS
b. ESS
c. IBSS
d. CBS
4. If a wireless access point is connected to a switch by a trunk port, which one of the following is mapped to a VLAN?
a. Channel
b. Frequency
c. BSS
d. SSID
5. If an RF signal meets a large metal filing cabinet, which one of the following effects is likely to occur?
a. Reflection
b. Refraction
c. Absorption
d. Scattering
CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter.
If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong. Giving yourself credit for an answer you correctly guess skews your self-assessment results and might give you a false sense of security.
“Do I Know This Already?” Quiz 433
6. Suppose that a wireless access point transmits at 100 mW, and then it is reconfigured to transmit at 50 mW. What is the change in dBm?
a. –50
b. 0.5
c. –3
d. +3
7. Antenna gain is usually measured in which one of the following units?
a. dBm
b. dBi
c. mW
d. W
8. A wireless access point transmits at 100 mW or 20 dBm. A cable with a loss of 5 dB connects the AP to its antenna. The antenna is a dish model with a gain of 22. The receiving access point is located 1 mile away and uses an identical antenna and cable. What is the EIRP value?
a. 0 dBm
b. 20 dBm
c. 37 dBm
d. 74 dBm
9. A wireless AP with a built-in dipole antenna would be best suited for which one of the following, based on its capability to cover the area?
a. Building-to-building link
b. Rooms located down a long hallway
c. An office made up of cubicles
d. A four-story office building
10. The IEEE 802.11g standard is backward compatible with which other standard?
a. IEEE 802.11a
b. IEEE 802.11b
c. IEEE 802.11c
d. IEEE 802.11e
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11. Which nonoverlapping channels are commonly used in wireless LANs?
a. 1, 2, 3
b. 1, 3, 6
c. 1, 6, 11
d. 1, 11, 22
12. Which one of the following represents the correct combination of frequency band and IEEE standard?
a. 2.4 GHz, 802.11g
b. 5 GHz, 802.11g
c. 2.4 GHz, 802.11a
d. 5 GHz, 802.11b
The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to Chapter ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:
■ 10 or less overall score—Read the entire chapter. This includes the “Foundation Topics,”
“Foundation Summary,” and “Q&A” sections.
■ 11 or more overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the “Q&A” section at the end of the chapter. Otherwise, move to Chapter 18, “Wireless Architecture and Design.”
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Foundation Topics
Wireless LAN Basics
This chapter presents wireless LAN (WLAN) operation from a practical viewpoint, building on the knowledge you’ve gained from the switched LAN topics. After all, this is a book (and exam) about switching technology, so you should know enough about wireless LANs to be able to integrate them into your switched network.
Comparing Wireless and Wired LANs
How exactly does a wireless LAN get integrated with a wired LAN? Where does switching fit into a wireless LAN? Before answering these questions, it might be helpful to see how the two technologies compare.
At the most basic level, switched networks involve wires, and wireless networks don’t. That might seem silly, but it points out some major differences in the physical layer.
A traditional Ethernet network is defined by the IEEE 802.3 standards. Every Ethernet connection must operate under tightly controlled conditions, especially regarding the physical link itself. For example, the link status, link speed, and duplex mode must all operate like the standards describe. Wireless LANs have a similar arrangement, but are defined by the IEEE 802.11 standards.
Wired Ethernet devices have to transmit and receive Ethernet frames according to the Carrier Sense Multiple Access/Collision Detect (CSMA/CD) method. On a shared Ethernet segment, where PCs communicate in half-duplex mode, each PC can freely “talk” first, and then listen for collisions with other devices that are also talking. The whole process of detecting collisions is based on having wired connections of a certain maximum length, with a certain maximum latency as a frame travels from one end of the segment to another before being detected at the far end.
Full-duplex or switched Ethernet links are not plagued with collisions or contention for the bandwidth. They do have to abide by the same specifications, though. For example, Ethernet frames must still be transmitted and received within an expected amount of time on a full-duplex link. This forces the maximum length of full-duplex, twisted-pair cabling to be the same as that of a half-duplex link.
Even though wireless LANs are also based on a set of stringent standards, the wireless medium itself is challenging to control. Generally speaking, when a PC attaches to a wired Ethernet
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network, it shares that network connection with a known number of other devices that are also connected. When the same PC uses a wireless network, it does so over the air. No wires or outlets exist at the access layer, as other end users are free to use the same air.
A wireless LAN then becomes a shared network, where a varying number of hosts contend for the use of the “air” at any time. Collisions are a fact of life in a wireless LAN because every wireless connection is in half-duplex mode.
Avoiding Collisions in a WLAN
When two or more wireless stations transmit at the same time, their signals become mixed.
Receiving stations can see the result only as garbled data, noise, or errors.
No clear-cut way exists to determine whether a collision has occurred. Even the transmitting stations won’t realize it because their receivers must be turned off while they are transmitting.
As a basic feedback mechanism, whenever a wireless station transmits a frame, the receiving wireless station must send an acknowledgement back to confirm that the frame was received error-free.
Acknowledgement frames serve as a rudimentary collision detection tool; however, it doesn’t work to prevent collisions from occurring in the first place.
The IEEE 802.11 standards use a method called Carrier Sense Multiple Access Collision Avoidance (CSMA/CA). Notice that wired 802.3 networks detect collisions, whereas 802.11 networks try to avoid collisions.
Collision avoidance works by requiring all stations to listen before they transmit a frame. When a station has a frame that needs to be sent, one of the two following conditions occurs:
■ No other device is transmitting—The station can transmit its frame immediately. The intended receiving station must send an acknowledgement frame to confirm that the original frame arrived intact and collision-free.
TIP IEEE 802.11 WLANs are always half-duplex because transmitting and receiving stations use the same frequency. Only one station can transmit at any time; otherwise, collisions occur.
To achieve full-duplex mode, all transmitting would have to occur on one frequency and all receiving would occur over a different frequency—much like full-duplex Ethernet links work.
Although this is certainly possible and practical, the 802.11 standards don’t permit full-duplex operation.
Wireless LAN Basics 437
■ Another device is already transmitting a frame—The station must wait until the frame in progress has completed, then it must wait a random amount of time before transmitting its own frame.
Wireless frames can vary in size. When a frame is transmitted, how can other stations know when the frame will be completed and the wireless medium is available for others to use? Obviously, stations could simply listen for silence, but doing so isn’t always efficient. Other stations can listen, too, and would likely decide to transmit at the same time. The 802.11 standards require all stations to wait a short amount of time, called the DCF interframe space (DIFS), before transmitting anything at all.
Transmitting stations can provide an estimate of the amount of time needed to send a frame by including a duration value within the 802.11 header. The duration contains the number of timeslots (typically in microseconds) needed for the size of frame being sent. Other wireless stations must look at the duration value and wait that length of time before considering their own transmissions.
Because every listening station receives and follows the same duration value found in a transmitted frame, every one of them might decide to transmit their own frames once the duration time has elapsed. This would result in a collision—the very condition that should be avoided.
In addition to the duration timer, every wireless station must also implement a random backoff timer. Before transmitting a frame, a station must select a random number of timeslots to wait.
This number lies between zero and a maximum contention window value. The idea here is that stations ready to transmit will each wait a random amount of time, minimizing the number of stations that will try to transmit immediately.
This whole process is called the Distributed Coordination Function (DCF), and is illustrated in Figure 17-1. Three wireless users have a frame to send at varying times. The following sequence of events occurs:
1. User A listens and determines that no other users are transmitting. User A transmits his frame and advertises the frame duration.
2. User B has a frame to transmit. He must wait until user A’s frame is completed, and then wait until the DIFS period has expired.
3. User B waits a random backoff time before attempting to transmit.
4. While user B is waiting, user C has a frame to transmit. He listens and detects that no one is transmitting. User C waits a random time, which is shorter than User B’s random time.
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5. User B transmits a frame and advertises the frame duration.
6. User C must now wait the duration of user B’s frame plus the DIFS time before attempting to transmit again.
Figure 17-1 Avoiding Collisions with the DCF Process
Because the backoff timer is random, a chance still exists that two or more stations will choose the same value. Nothing else will prevent these stations from transmitting at the same time and causing a collision. This will simply be seen as an error over the wireless network; no acknowledgements will be returned, and the stations will have to reconsider sending their frames again.
Finally, what if a station waits until its random backoff timer expires and is ready to transmit, only to find that someone else is already transmitting? The waiting station must now wait the duration of the newly transmitted frame, followed by the DIFS time, and then the random backoff time.
WLAN Building Blocks
At the most basic level, a wireless medium has no inherent organization. For example, a PC with wireless capability can simply bring up its wireless adapter anywhere at any time. Naturally, there must be something else that can also send and receive over the wireless media before the PC can communicate.
TIP In IEEE 802.11 terminology, any group of wireless devices is known as a service set. The devices must share a common service set identifier (SSID), which is a text string included in every frame sent. If the SSIDs match across the sender and receiver, the two devices can communicate.
User A
User B
User C
Frame (A)
Frame (C)
Frame (B)
Duration DIFS
Random Backoff
DIFS Duration
Duration User B is ready to
send a Frame, but WLAN is busy.
Random Backoff
DIFS
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The PC, as an end-user station, becomes a client of the wireless network. It must have a wireless network adapter and a supplicant, or software that interacts with the wireless protocols.
The 802.11 standards allow two or more wireless clients to communicate directly with each other, with no other means of network connectivity. This is known as an ad-hoc wireless network, or an Independent Basic Service Set (IBSS), as shown in part A of Figure 17-2.
Figure 17-2 A Comparison of Wireless Service Sets
No inherent control exists over the number of devices that can transmit and receive frames over a wireless medium. As well, many variables exist that can affect whether a wireless station can
A IBSS
Access Point
B BSS
Access Point Access Point
C ESS
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receive from or transmit to other stations. This makes providing reliable wireless access to all stations difficult.
An 802.11 Basic Service Set (BSS) centralizes access and control over a group of wireless devices by placing an access point (AP) as the hub of the service set. Any wireless client attempting to use the wireless network must first arrange a membership with the AP. The AP can require any of the following criteria before allowing a client to join:
■ A matching SSID
■ A compatible wireless data rate
■ Authentication credentials
Membership with the AP is called an association. The client must send an association request message, and the AP grants or denies the request by sending an association reply message. Once associated, all communications to and from the client must pass through the AP, as shown in part B of Figure 17-2. Clients can’t communicate directly with each other, as in an ad-hoc network or IBSS.
A wireless AP isn’t a passive device like an Ethernet hub, however. An AP manages its wireless network, advertises its own existence so that clients can associate, and controls the communication process. For example, recall that every data frame sent successfully (without a collision) over a wireless medium must be acknowledged. The AP is responsible for sending the acknowledgement frames back to the sending stations.
Notice that a BSS involves a single AP and no explicit connection into a regular Ethernet network.
In that setting, the AP and its associated clients make up a standalone network.
An AP can also uplink into an Ethernet network because it has both wireless and wired capabilities. If APs are placed at different geographic locations, they can all be interconnected by a switched infrastructure. This is called an 802.11 Extended Service Set (ESS), as shown in part C of Figure 17-2.
In an ESS, a wireless client can associate with one AP while it is physically located near that AP.
If the client later moves to a different location, it can associate with a different nearby AP. The TIP Keep in mind that regardless of the association status, any PC is capable of listening to or receiving the frames that are sent over a wireless medium. Frames are freely available over the air to anyone who is within range to receive them.
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802.11 standards also define a method to allow the client to roam or to be passed from one AP to another as its location changes.
Access Point Operation
An AP’s primary function is to bridge wireless data from the air to a normal wired network. An AP can accept “connections” from a number of wireless clients so that they become members of the LAN, as if the same clients were using wired connections.
An AP can also act as a bridge to form a single wireless link from one LAN to another over a long distance. In that case, an AP is needed on each end of the wireless link. AP-to-AP or line-of-sight links are commonly used for connectivity between buildings or between cities.
Cisco has developed an AP platform that can even bridge wireless LAN traffic from AP to AP, in a daisy chain fashion. This allows a large open outdoor area to be covered with a wireless LAN but without the use of network cabling. The APs form a mesh topology, much like an ESS where APs are interconnected by other wireless connections.
APs act as the central point of access (hence the AP name), controlling client access to the wireless LAN. Any client attempting to use the WLAN must first establish an association with an AP. The AP can allow open access, so that any client can associate, or it can tighten control by requiring authentication credentials, or other criteria before allowing associations.
The WLAN operation is tightly coupled to feedback from the far end of a wireless connection.
For example, clients must handshake with an AP before they can associate and use the WLAN.
At the most basic level, this assures a working two-way wireless connection because both client and AP must be able to send and receive frames successfully. This process removes the possibility of one-way communication, where the client can hear the AP, but the AP can’t hear the client.
As well, the AP can control many aspects of its WLAN by requiring conditions to be met before clients can associate. For example, the AP can require that clients support specific data rates, specific security measures, and specific credentials during client association.
You can think of an AP as a translational bridge, where frames from two dissimilar media are translated and then bridged at Layer 2. In simple terms, the AP is in charge of mapping a VLAN to an SSID. This is shown in the left portion of Figure 17-3, where VLAN 10 on the wired network is being extended to the AP over a switch port in access mode. The AP maps VLAN 10 to the wireless LAN using SSID “Marketing.” Users associated with the “Marketing” SSID will appear to be connected to VLAN 10.